Load balancer monitor additional host header not saved

I added a Host header to the load balancer monitor in advanced setting and saved it. But when I open the edit form again it is gone. How do I know if the Host header is set on the monitor? I checked through the API and saw no header as well…

The reason I want to set the header is that when I set up the load balancer and enable it the main pool is shown as critical with the error:

TLS name mismatch error

In the documentation and this question I found that you need to set the header for it to be fixed, but after I set the header nothing changes…